Intensified processing methodologies are used increasingly in place of traditional chemical processing routes when small production volumes are warranted or when portability of process equipment is desired. For example, large scale hydroprocessing of biofuels derived from local sources is impractical for isolated military units in remote locations. Instead, portable conversion units and/or units with small footprints are needed. However large scale processing reactors are not easily downscaled for such uses. Thus, the need exists for competent catalysts and corresponding reactor configurations that are suitable for on-site process chemistry in small, modular units.
Simply operating established heterogeneous catalysts in compact reactor configurations at higher space velocities and at more severe temperatures than used in traditional processing will not necessarily increase productivity as needed. Under such modified conditions, mass and heat transfer limitations attenuate maximum catalyst activity, especially when such processes are conducted in traditional reactor designs. Consequently, intrinsically fast catalytic cycles alone afford no additional productivity benefits without addressing the mass and heat transfer limitations.
A variety of different shapes and styles of heterogeneous catalysts are currently available that seek to augment the ratio of geometric surface area to occupied reactor volume as a means of mitigating mass and heat transfer limitations. Preformed metallic scaffolding structures are preferred over ceramic scaffolding structures when very thin walls or low pressure drop is needed in densely packed channel structures such as those desirable for intensified processing. In particular, microchannel constructs, such as thin-walled metallic honeycombs, covered with a minimally thick catalytic layer, have been sought to decrease resistance between process fluids and channel walls thereby promoting rapid convective heat and mass transfer as well as conductive heat transfer.
The ratio of surface atoms exposed to process fluids to total atoms of a catalytically active metal cluster is termed “dispersion” for the purposes of this discussion. Catalyst support surfaces that enable high dispersion of applied catalytic agents are sought in the art. To reduce required content of costly catalytic components and to achieve sufficiently high dispersion to promote high catalytic activity per structural unit, base metal or precious or platinum group metal heterogeneous catalysts usually are applied to catalyst support materials, often composed of high surface area oxide powders. Typically, structured catalyst supports are produced by applying an intermediate washcoat layer of inert metal oxide or aqueous hydroxide slurries directly onto metal foils that have been preformed into a desired scaffolding structure. A second application of precursors transformable to reduced metal clusters, e.g. alcoholic or aqueous solutions of metal salts, is usually applied afterward. Alternatively, the metal salts can be admixed with the oxide or hydroxide aqueous slurries and applied in one step to the metal substrate. Catalytically active metals or active compounds are thereby distributed within the washcoat layer, rather than in exclusive direct contact with bare metal substrate. The active material therefore sits on the surface of the intermediate washcoat layer and is insulated from the underlying metallic substrate. As such, the washcoating is susceptible to damage, such as delamination, from aggressive physical manipulation and/or intensified process conditions because the coating is only weakly adhered to the underlying metal scaffolding structure. These catalyst structures usually are coated after physical forming of the scaffold to minimize damage to the cured catalyst coatings that could result if mechanical processing were done after application of the washcoat.
The application of slurries to preformed substrates also can result in the coating having a nonuniform distribution. Preformed substrates are normally dip-coated or spray-coated with the washcoat slurry, and excess slurry is removed using an air knife. Excess slurry is difficult to remove from small crevices and corners, particularly in catalyst structures containing microchannels, and can result in varying thicknesses of the washcoat throughout the catalyst structure, which leads to a catalyst layer of varying thickness on the substrate.
When such a supported catalyst is used to accelerate an exothermic reaction, e.g. catalytic oxidation of entrapped soot particles or of gaseous hydrocarbons, the varying thickness of the catalyst could result in hot spots forming in the catalyst layer, which in turn can cause melting of the substrate or sintering of the active phase, thereby prematurely reducing dispersion and corresponding activity. An alternative to the use of high surface area supported base metals as catalysts is the possibility for use of bulk skeletal metal aggregates, such as Raney metals, to prepare highly active catalysts. These skeletal metal particles typically are used in slurry phase processing or, less commonly, in packed beds. The latter usually suffer from pressure drop or particulation problems in practical use. Small channel monolith structures containing bound bulk skeletal metal catalysts, which in principle could generate a diminished pressure drop compared to packed beds under high space velocity conditions, would be difficult and costly to fabricate. Furthermore, under severe process conditions, such as encountered in steam methane reforming for example, bulk skeletal metal aggregates would rapidly deactivate due to surface sintering or easily delaminate from their underlying scaffolding, if used.